In recent years, in view of global environmental conservation, it has been strongly required that automobiles are improved in fuel efficiency. Therefore, the drastic weight reduction of the bodies of automobiles and the like is demanded. Even structural parts of automobiles and the like are no exception. To achieve a good balance between weight reduction and safety, high-strength electrically welded steel tubes are used for some of the structural parts. Conventional electrically welded steel tubes used as raw materials have been formed so as to have a predetermined shape and then subjected to thermal refining such as quenching, whereby high-strength structural parts have been obtained. However, the use of thermal refining causes the following problems: an increase in the number of production steps, an increase in the time taken to produce structural parts, and an increase in the production cost of the structural parts.
To cope with the problems, Japanese Patent No. 2588648 discloses a method of producing an ultra-high tensile strength electrically welded steel tube for structural parts of automobiles and the like. In the method disclosed in Japanese Patent No. 2588648, a steel material in which the content of C, Si, Mn, P, S, Al, and/or N is appropriately adjusted and which contains 0.0003% to 0.003% B and one or more of Mo, Ti Nb, and V is finish-rolled at a temperature ranging from its Ar3 transformation point to 950° C. and is then hot-rolled into a steel strip for tubes in such a manner that the steel material is coiled at 250° C. or lower, the steel strip is formed into an electrically welded steel tube, and the electrically welded steel tube is aged at a temperature of 500° C. to 650° C. According to the method, an ultra-high tensile strength steel tube having a tensile strength of greater than 1000 MPa can be obtained without performing thermal refining because of transformation strengthening due to B and precipitation hardening due to Mo, Ti, and/or Nb.
Japanese Patent No. 2814882 discloses a method of producing an electrically welded steel tube suitable for door impact beams and stabilizers of automobiles and which has a high tensile strength of 1470 N/mm2 or more and high ductility. In the method disclosed in Japanese Patent No. 2814882, the electrically welded steel tube is produced from a steel sheet made of a steel material which contains 0.18% to 0.28% C, 0.10% to 0.50% Si, 0.60% to 1.80% Mn, 0.020% to 0.050% Ti, 0.0005% to 0.0050% B, and one or more of Cr, Mo, and Nb and in which the amount of P and S is appropriately adjusted; is normalized at a temperature of 850° C. to 950° C., and is then quenched. According to this method, an electrically welded steel tube having a high strength of 1470 N/mm2 or more and a ductility of about 10% to 18% can be obtained. This electrically welded steel tube is suitable for door impact beams and stabilizers of automobiles.
An electrically welded steel tube produced by the method disclosed in Japanese Patent No. 2588648 has a small elongation El of 14% or less and low ductility and therefore is low in formability; hence, there is a problem in that the tube is unsuitable for automobile structural parts, such as torsion beams and axle beams, made by press forming or hydro-forming.
An electrically welded steel tube produced by the method disclosed in Japanese Patent No. 2814882 has an elongation El of up to 18% and is suitable for stabilizers formed by bending. However, this tube has ductility insufficient to produce structural parts by press forming or hydro-forming. Therefore, there is a problem in that this tube is unsuitable for automobile structural parts, such as torsion beams and axle beams, made by press forming or hydro-forming. Furthermore, the method disclosed in Japanese Patent No. 2814882 requires normalizing and quenching, is complicated, and is problematic in dimensional accuracy and economic efficiency.
It could therefore be helpful to provide a high-tensile strength welded steel tube which is suitable for automobile structural parts such as torsion beams and which is required to have excellent torsional fatigue endurance after the tube is formed into cross-sectional shape and is then stress-relief annealed. It could also be helpful to provide a method of producing an electrically welded steel tube for structural parts of automobiles without performing thermal refining. This tube would have a yield strength of greater than 660 MPa, excellent low-temperature toughness, excellent formability, and excellent torsional fatigue endurance after this tube is formed into cross-sectional shape and is then stress-relief annealed.